opinion:100lb is a safety factor for them. your only suppose to fill 80% capacity. by the way; what happens hen it gets 3/4 empty? I used 20lb tank and lasted about 8hrs on 40,00btu model in 20F weather no insulation to speak of in bus when first starting.--was insulating bus in winter. Propane is around 4.4 or 4.6 lb per gal if memory serves me correct and a standard grill tank holds under 4 gal. aprox. FWIW

I have two of these,, they work well. Allways crack a window when used in an enclosed area to replace oxygen that is displaced from the combustion process,, they all have and advertise an "oxygen depletion sensor",, in fact it is only a thermocouple that shuts off when the oxygen level gets too low to support a flame,, by then its dangerously low.>>>Dan

I wonder if one of the issues is a smaller tank freezing up? Our Boy Scout troop uses mostly 5 lb propane tanks. We used a 5 lb tank with a high BTU three burner stove once. The 5 lb tank eventually froze up and we had to switch to a 20 lb tank.

We have a 10000 btu unit in our 1000 sqft mobile home. It's kept our home at 70 inside when it got down to 25 outside. Propane line is routed thru the floor outside. We run it off of 20 lb tanks without a problem. One tank last approx. 52 hrs on high. Plan on putting a similar thermostat controlled model in the bus when i get to building the interior. Hope this helps.

the freezing up issue will usually crop up after tank gets to about 1/2 if you are drawing enough. As it starts freezing, the pressure starts dropping, and I imagine it could get to the point of incomplete combustion or flame out with gas seeping out after pressure starts to climb from tank normalizing.(thawing) Less chance of a problem with the big tank.